The use of molecular sieves as catalysts in aromatic conversion processes are well known in the chemical processing and refining industry. Aromatic conversion reactions of considerable commercial importance include the alkylation of aromatic compounds such as in the production of ethyltoluene, xylene, ethylbenzene, cumene, or higher alkyl aromatics and in disproportionation reactions such as toluene disproportionation, xylene isomerization, or the transalkylation of polyalkylbenzenes to monoalkylbenzenes. Often the feedstock to such an aromatic conversion process will include an aromatic component, i.e. alkylation substrate, such as benzene, and a C2 to C20 olefin alkylating agent or a polyalkyl aromatic hydrocarbon transalkylating agent. As used herein, terms such as “C4”, “C5”, “C6”, etc. designate the number of carbon atoms per molecule of a hydrocarbon or hydrocarbon specie. In the alkylation zone, the aromatic feed stream and the olefinic feed stream may be reacted over an alkylation catalyst to produce alkylated aromatics, e.g. cumene or ethylbenzene. A portion or all of the alkylation substrate may be provided by other process units including the separation section of a styrene process unit. Polyalkylated benzenes are separated from monoalkylated benzene product and recycled to a transalkylation zone and contacted with benzene over a transalkylation catalyst to yield monoalkylated benzenes and benzene.
Catalysts for aromatic conversion processes generally comprise zeolitic molecular sieves. Examples include, zeolite beta (U.S. Pat. No. 4,891,458); zeolite Y, zeolite omega and zeolite beta (U.S. Pat. No. 5,030,786); X, Y, L, B, ZSM-5 and Omega crystal types (U.S. Pat. No. 4,185,040); X, Y, ultrastable Y, L, Omega, and mordenite zeolites (U.S. Pat. No. 4,774,377); and UZM-8 zeolites (U.S. Pat. Nos. 6,756,030 and 7,091,390). It is known in the art that the aromatic feed stream to aromatic conversion processes often contains nitrogen compounds, including weakly basic organic nitrogen compounds such as nitriles that can, even at ppm and ppb levels, cumulatively act to poison the downstream aromatic conversion catalysts such as aromatic alkylation catalysts and significantly shorten their life. A variety of guard beds having clay, zeolite, or resin adsorbents to remove one or more types of basic nitrogen compounds from an aromatic hydrocarbon stream upstream of an aromatic conversion process are known in the art. Examples include: U.S. Pat. Nos. 7,205,448; 7,744,828; 6,297,417; 5,220,099; WO 00/35836; WO 01/07383; U.S. Pat. Nos. 4,846,962; 6,019,887; and 6,107,535.
Water is often found in the aromatic feedstock to alkylation and transalkylation reactions, especially in benzene feed. Benzene feed is often water saturated, for example, when it is recycled from a styrene monomer unit. Molecular sieve catalysts employed in alkylation reactions in the vapor or the liquid phase may be sensitive to water at various levels or sulfur compounds in the feedstock. U.S. Pat. No. 4,107,224 discloses that water and hydrogen sulfide in vapor phase reactions may be tolerable if more rapid aging of the catalyst is acceptable. U.S. Pat. No. 5,030,786 disclose the dehydration of the feedstock to a water content of no more than 100 ppm, and preferably 50 ppm or less when the reaction zone is operated to maintain the reactor contents in the liquid phase. However, WO 93/00992 discloses that in the starting phase the zeolite catalyst for alkylation or transalkylation processes should have a minimum water content of more than 3.5 wt-%, related to catalyst composition. EP 0 922 020 B1 discloses uses of a solid acid to adsorb impurities from a benzene alkylation feed which is dried to contain no more than 200 ppm water at a temperature of between 130° and 300° C. to improve the lifetime of a zeolitic alkylation or transalkylation catalyst.
Other impurities present in the feedstock to an aromatic conversion reactor, particularly basic impurities such as basic organic nitrogen compounds (ONCs), neutralize the solid acids that comprise most present day aromatic alkylation catalysts. Catalyst performance and the catalyst life are adversely affected. Even very low nitrogen concentrations in the feed increase the catalyst regeneration frequency during which accumulated nitrogen compounds and coke must be combusted from the catalyst. As more active zeolite catalysts are employed in aromatic conversion reactions, the degradation of catalyst life by nitrogen impurities in the feedstock must be more carefully controlled. Processes are sought to reduce the impact of nitrogen impurities on the catalyst in the reaction zone. Basic nitrogen compounds that degrade catalyst life include indoles, pyridines, quinolines, diethanol amine (DEA), morpholines including N-formyl-morpholine (NFM) and N-methyl-pyrrolidone (NMP). NFM and NMP are used as aromatic extraction agents and DEA is a corrosion inhibitor that all often contaminate aromatic feed streams. Sacrificial guard beds have been used upstream of hydrocarbon conversion catalysts in order to remove the basic nitrogen compounds to protect the hydrocarbon conversion catalyst. For instance, U.S. Pat. No. 5,220,099 teaches removing indole, quinoline and pyridine impurities with zeolites and using toluene with dissolved water to desorb the impurities from the zeolites. WO 00/35836 discloses contacting an alkylated benzene with molecular sieve to remove catalyst poisons including nitrogen compounds prior to feeding it to a transalkylation reactor. WO 01/07383 discloses contacting a feed stream to an alkylation zone with a zeolite to remove organically bound nitrogen. U.S. Pat. No. 4,846,962 discloses contacting a solvent extracted oil with an amorphous silica-alumina or crystalline zeolite adsorbent to remove basic nitrogen compounds such as NMP. The adsorbent may contain up to 30 wt-% water.
U.S. Pat. No. 5,271,835 discloses the presence of polar impurities in the C3 to C5 product fraction from a fluid catalytic cracking unit. The impurities were found to include weakly basic ONCs such as acetonitrile. Acrylonitriles and propionitrile can also be found in hydrocarbon streams that may serve as feed to an aromatic alkylation process. While basic nitrogen compounds are readily removed from hydrocarbon feed stream by even weakly acidic adsorbents, weaker bases, like weakly basic nitrogen compounds are more difficult to remove. Unfortunately, these weakly basic nitrogen compounds are attracted to and poison the catalyst used in aromatics alkylation processes and may cause alkylation catalyst deactivation. Even low concentrations of nitriles in the ranges of parts per million and parts per billion can cumulatively deactivate alkylation catalysts faster than other deactivation mechanisms such as coking. Weak bases typically require strongly acidic materials for their effective removal from a hydrocarbon feed stream. Thus, while clay or resin guard beds are inexpensive means to adsorb basic nitrogen compounds from aromatic alkylation feed streams, more expensive acidic zeolites are typically used to remove weakly basic nitrogen compounds as they have a higher acid site strength and density than other materials, such as acidified clays.
U.S. Pat. No. 6,019,887 teaches using a cationic nonacidic zeolite at no more than 300° C., and U.S. Pat. No. 6,107,535 teaches using silica gel to adsorb nitriles at room temperature from a hydrocarbon stream. U.S. Pat. No. 2,999,861 teaches using an X zeolite to selectively adsorb basic
ONCs over weakly basic ONCs including nitriles, nitrates and nitro compounds at −18 to 427° C. U.S. Pat. Nos. 5,744,686 and 5,942,650 teach removing water from a benzene stream containing nitriles before removing the nitriles by contacting the benzene stream with nonacidic molecular sieves at −18° to 204° C. U.S. Pat. No. 6,617,482 B1 teaches higher silica zeolites are more effective when water is present. However, only adsorption of NFM in the presence of water is demonstrated at room temperature; adsorption of nitriles is demonstrated only in the absence of water in this reference.
It is desirable, therefore, to provide less expensive means for removing nitrogen compounds, including weakly basic nitrogen compounds, from hydrocarbon feed streams, while effectively removing the nitrogen compounds from the feed stream to prolong the life of more expensive downstream catalysts, such as alkylation catalysts.